This invention relates generally to a power transfer mechanism adapted to be incorporated in a multiple path drive system, for example a four-wheel drive vehicle. More particularly, it relates to a torque transfer case adapted to receive torque from a prime mover and to provide torque for one drive axle when two-wheel drive is adequate and to provide torque for both drive axles automatically when four-wheel drive is required. The torque transfer case incorporates a double-acting overrunning clutch for automatically engaging and disengaging the four-wheel drive function.
In recent years there have been many improvements in automotive drive trains, including improvements relating to the transfer of torque from a prime mover to drive axles. Where four-wheel drive systems are used, transfer cases have been developed which direct torque to one output to drive a fixed axle and through an overrunning clutch to another output for driving a steering axle. One such transfer case is disclosed in U.S. Pat. No. 3,993,152 issued Nov. 23, 1976, wherein an overrunning clutch automatically engages and disengages the four-wheel drive function. However, under certain conditions undesirable clutch lockup may occur, thus negating its automatic function. This problem now is believed to have been solved by the direction-sensitive blocking structure disclosed in U.S. application No. 716,629 filed Aug. 23, 1976.
Under certain operating conditions there is a tendency for the fixed axle wheel to skid, with consequent loss of vehicle control. As disclosed in U.S. Application No. 676,774 filed Apr. 14, 1976, this tendency may be corrected by providing sufficient drive back from the steering axle wheels to keep the fixed axle wheels rotating. A drive path is established between the axles by loosely connecting one clutch race element with the steering axle, and by providing frictional engagement between this element and the fixed axle. However, it is now known that such a drive path is not fully independent of the clutch and may interfere with clutch operation.